It is known that double metal cyanides, generally known as DMC catalysts, can be used to catalyse alkene oxide polyaddition reactions in substances that contain active hydrogens (see, e.g. the patents U.S. Pat. No. 3,404,109, U.S. Pat. No. 3,941,849 and U.S. Pat. No. 5,158,922). The use of these DMC catalysts in the production of polyether polyols permits achieving, specifically, a reduction in the content of monofunctional polyether polyols with a terminal double bond, in comparison with those polyether polyols produced by conventional polyether polyol production processes using alkaline catalysts, e.g. potassium hydroxide. The polyether polyols thus obtained permit the production of polyurethanes with improved properties for their use as elastomers, foams, etc.
DMC catalysts are usually produced by the treatment of aqueous solutions of metal salts with aqueous solutions of metal cyanide salts in the presence of organic ligands with low molecular weights, e.g. ethers. In a typical preparation of these catalysts, an aqueous solution of zinc chloride (in excess) is mixed with an aqueous solution of potassium hexacyanocobaltate with dimethoxyethane (diglyme) to form a suspension. After separating the solid catalyst by filtering and washing it with an aqueous solution of diglyme, active catalysts with general formula Zn3[Co(CN)6]2.xZnCl2.yH20.zDiglyme (see, e.g. EP 700 949), are produced. The patent applications EP 700 949, WO 97/40086 and WO 98/16310 disclose improved DMC catalysts that use functionalized polyether polyols or polymers in addition to double metal cyanide and the organic ligand. These improved DMC catalysts have high activity and permit the production of polyether polyols with low concentrations of catalytic residues (20-25 p.p.m, see Table 1 in WO 98/16310). Patent application WO 99/19063 discloses crystalline double metal cyanides, which are high activity catalysts to produce polyether polyols. In the patent U.S. Pat. No. 5,844,070 a rapid activation process of DMC catalysts is disclosed.
Likewise, various processes have been disclosed for the elimination of catalytic residues in polyether polyols produced with DMC catalysts (see, e.g. the patents U.S. Pat. No. 4,355,188, U.S. Pat. No. 4,721,818, U.S. Pat. No. 4,877,906, U.S. Pat. No. 4,987,271, U.S. Pat. No. 5,010,047, EP 0385 619, U.S. Pat. No. 5,099,075, U.S. Pat. No. 5,144,093, U.S. Pat. No. 5,248,833, U.S. Pat. No. 5,416,241, U.S. Pat. No. 5,416,241 and U.S. Pat. No. 5,973,096. Although these processes are effective for the elimination of the catalytic residues of polyether polyols produced with DMC catalysts, they generally require the use of additional reagents and relatively complicated operations. U.S. Pat. No. 4,843,054, proposes the preparation of filterable DMC catalysts suitable for the polymerization of propylene oxide. For this, the reaction of zinc chloride and potassium hexacyanocobaltate is carried out in water-diglyme in the presence of an inert filtration coadjuvant. In said patent, alumina, silica gel, aluminium silicate, magnesium silicate, diatomaceous earth, perlite, carbon black, carbon, etc are mentioned as inert filtration coadjuvants. That patent therefore relates to an improved process to prepare DMC catalysts.
It can be appreciated that the art has been searching for increasingly DMC catalysts to reduce the concentration of catalytic residues in the polyols and thus be able avoid tedious, costly processes to eliminate the catalytic residues. Nevertheless, at the present time, there is a need for polyols with even lower catalytic residues and which are also free from other impurities, which give the polyether polyols undesirable characteristics. In this way, it is known that the polyols produced by the aforementioned processes, and the polyurethanes produced with them, generally have a very unpleasant odour. Although this odour does not negatively affect the chemical properties of the polyether polyols, it would be desirable to have polyols without said unpleasant odours. The known processes to eliminate these odours are complicated and costly (see, e.g. patent application WO 99/47582).